The subject invention, which was made under or in the course of a contract with the United States Air Force, pertains generally to electronics communications and particularly FSK demodulators.
A conventional technique for retrieving binary data transmitted as a frequency shifted keyed (FSK) signal is to separate the signal into the two frequency channels which define the mark and space information of each bit, normally logic states "1" and "0", respectively. The bandwidth of the individual channel filters which effectuate the separation should be wide enough to pass the data at the rate employed but sufficiently narrow to restrict noise components which would otherwise degrade performance and increase the bit error rate. With such a design, gain imbalance between the mark and space channels resulting from differing characteristics of like components in the individual channel paths is not usually a problem. Ideally, of course, equal gain in both channels is desired since then the noise contained in the channel outputs is cancelled so as not to introduce any unwanted bias when the outputs are combined to replicate the data bit stream. However, when the channel filter bandwidths have to be expanded beyond that necessary to accommodate the data rate, for example the Doppler effect when the communications equipment is located on a moving vehicle, the resultant gain imbalance effect on the noise components which are then enabled to pass through the channels may not be ignored since it can introduce a bias which materially contributes to bit error rate in the demodulation process. Although gain imbalance can be corrected by initial adjustment of the individual channel components under specified test conditions at a particular ambient temperature, temperature changes thereafter during operation may result in characteristic differences sufficient to produce unacceptable gain imbalance.
With the foregoing in mind, it is a primary object of the present invention to afford automatic channel gain imbalance compensation in an FSK demodulator.
It is a further object of the present invention to provide such channel gain imbalance compensation through a simple yet effective means.
The foregoing objects, as well as others, and the means by which they are achieved, may best be appreciated by referring to the Detailed Description of the Preferred Embodiment which follows hereinafter together with the single appended drawing which depicts the invention.